Although many lines of evidence document the importance of maintaining a relatively high calcium intake during pubertal development, the calcium intake of as many as 20% of pubertal American girls is less than one-third of the currently recommended level. Virtually no information is available regarding the physiological adaptation to these low intakes. For example, despite a lower average calcium intake, peak bone mass is greater and the risk of osteoporosis is lower in African-American compared with Caucasian girls. This proposal will evaluate the adaptation to low calcium intakes by girls during puberty and compare the differences in that adaptation between Caucasian and African-American girls. The specific aims of the project have not changed during the initial year since the project began. To summarize, these were to: 1) To assess the effects of decreasing calcium intake to 400 mg/day in girls on calcium absorption, excretion and bone formation/resorption; 2)To determine the changes in calcium retention and kinetics after supplementing the intake of girls with very low habitual intakes of calcium (Group 2) with 600 mg of calcium for 6 months; 3) To evaluate racial differences in adaptation to low calcium intakes which may partially account for the lower incidence of osteoporosis in African-Americans compared to Caucasians despite the lower calcium intake of African-Americans; 4) To determine the relationship between hormonal and pubertal status and adaptation to low calcium intake in healthy girls. The study was designed to evaluate 32 girls (16 White and 16 Black) girls each with natural low or high intakes (total of 64 girls). Each girl is to be evaluated twice. To date, we have confirmed the recruitment and scheduling of 24 of the 64 girls (38% of the total) to have their initial studies completed by April 1999. O f these, initial studies have already been completed (by 02/01/99) on 14 of the girls and preliminary results are available from these initial studies on 8 of these 14. Results from these 8 studies show that greater calcium absorption was present on a low vs. a high diet with an absorption maximum of approximately 65% on the low calcium diets. We have not identified any problems or limitation in perfor4ming the studies. There has not been any change in study personnel. At this time date are not available to provide an interpretation of the study finding in relationship to the specific aims. Evidence of increased calcium absorption on low calcium diets is expected and will need to be related to dietary intake levels and hormonal findings.